If this car has a belt driven water pump as most cars do, driving with a snapped belt will cause severe overheating which can kill any engine, let alone one with a turbo.
If that is the case, the problem is not BMW.
A friend has driven 5-series BMWs for decades. He loves them, and the problems are not a big deal to him. Preventive maintenance is how he keeps them from having expensive problems. In what heās learned from experience and participation in BMW forums, heās set up a plastic and rubber parts replacement schedule to avoid failures. The extra work is worth it to him to have the cars he likes best.
My daughter has a BMW of similar vintage. When it had about 65,000 on it, the engine made a strange noise. She was near the dealership when it happened and pulled onto the lot. The engine died and would not start. They told her the timing belt tensioner had failed and the damage was extensive and a new engine was $25,000. She had an extended warranty that expired at 60,000, and, while on full warranty, was recalled for a timing belt tensioner replacement. After some discussion, BMW made a concession and replaced the engine for $5,000.
Once again, the next piece of wisdom is worth highlighting:
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If the info pulled up is correct it shows this model uses a timing chain, not a belt, and the water pump is electric.
With an engine tap this kind of takes me back to my query about wondering what the motor oil level is. With no dipstick then one has to wonder if any low oil level warning was ignored or whether or not that feature is inoperative on the car. Just theorizināā¦
This BMW would have no dip stick, but an electronic sensor.
If oil went low, driver would have received a warning on his dash, so I would not think scenario of neglecting to check oil level is very likely.
The link I provided above makes a case that it is ātypicalā for these engines to get into the oil starvation condition after serpentine belt is snapped, due to front seal damage and rubber plugging pickup tube, so it is quite likely to get tapping symptoms even with full oil pan.
The engine is a toast one way or anotherā¦
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ā¦ assuming that the sensor is still working, along with the warning light on the instrument panel. Although I (and presumably, you) periodically check whether all of our warning lights are still functioning, I suspect that many/most people donāt do so.
on 5-years old car, I would assume that sensor still works
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Yes, probably, but we are all left with a lot of assumptions in this situation, and only the OP could clarify matters for us.
Yes there are assumptions made which can only be clarified by the owner of the car. Yes Iām aware that the BMW has no motor oil dipstick.
An assumption is made that the car is only 5 years old so the oil level sensor is likely good. Maybe, maybe not.
Here are some more assumptions. The OP bought the car with 50k miles on it and added another 25k miles for a total of 75k.
What if the oil change regimen during that first 50k (or even the last 25k) was āsparseā at best? What if the engine was sludged up horribly, along with the oi level sensor, and a Low Oil Level message was never shown because of that sludged up sensor?
Another assumption but if the theory about a serpentine belt ruining the front seal was what actually happened then the front of the engine should be covered in oil and the Low Oil Message should have appeared; but may have been ignored. How many people ignore oil pressure lamps and temp gauges; along with ball joint noises, grinding brakes, slipping clutches, ad nauseum.
The BMW dealer says it needs a new engine and I take them at their word.
Just some food for thought.
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Yes, indeed, a lot of assumptions in absence of further info from @galm26
+1
Additionally, if the correct āEuro-specā oil wasnāt used, then the chance of a badly-sludged engine increases exponentially, IMHO.
And, when buying a used BMW, most people probably wouldnāt even know about the importance of verifyingāwith hard copy documentationāthat the correct-spec oil was used. 50k of āextendedā oil changes with the wrong-spec oil could destroy that engine.
back to @db4690 point? 
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If the problem with this BMW is engine oil sludge or coking then the odds of neglect are about 90% IMO. To me anyway, itās premature and a knee jerk reaction to blame the car for this problem as no info is provided other than a serpentine belt shredded and the engine is tapping.
This may or may not be a BMW fault. In the event of oil sludging or coking I can say that Iāve seen just about everything in the automotive inventory afflicted with this problem. GM, Ford, Chrysler, Nissan, Subaru, Honda, Toyota, VW, Fiat, you name it.
Many cars are routinely trashed at far less than the mileage at which point the OP bought this one; which was 50k miles. Iāve seen cars come in with under 20k miles suffering engine damage because of few if any oil changes, never checking the oil level, or a botched fast lube oil change in which the car gets towed in with fresh oil/filter and the customer playing dumb while insisting on waranty.
+1
It is absolutely not BMW fault the belt shredded, it is total neglect by the owner.
BMW may be chuckled upon for why shredded belt may lead to so much trouble (same as Volvo), but the root cause is indeed the ownerā neglect.