Slipping Transmission

My son changed the oil in our 97 Lexus. (It has about 185K miles) The transmission started slipping same day. Any suggestions?

I had a friend once that accidentally drained the trans fluid when changing the oil.

There’s a lot not known but it could be a coincidence at that many miles or due to other factors. If it’s slipping then it’s probably on the last legs.

How many times has he changed the oil on this car and was he driving it when the transmission started acting up?

Any chance he drained the transmission by mistake and the engine oil is overfilled? The Lexus should have a drain plug on the transmission and could possibly be mistaken for an engine drain; all depending.

I bet he drained the transmission as others have suggested