Any leaks, foul fuel if it be sitting, old plastics and rubbers including coolant & vacuum hoses, tires, suspension components if not properly maintained throughout the years this far (aka been sitting), highly recommend to check out oil pan and condition of all the filters. A/C is probably toast so don't expect it unless you got a miracle. Few and far between. Electrical gremlins. Rust. Si panels are a big one. That is like your ODO, and dash in general and use a hella convoluted old AF battery that is always dead. Speaking of which, dash gears and dials not working.

You know just old car things LOL. I have a 1987 BMW 325IS and these are just a few things that I have had to do. Mine was sitting for 8 years before I got it. I just ripped out my AC fOr ThE wEiGhT rEdUcTiOn obviously haha. Live in California, much not use anyway.

If he is having to ask about repairs, just get rid of it and get something else. Yes they were better back then, but they still break and are still expensive to fix.

You didn’t state which engine or mileage, but see below for some issues that he could encounter:

http://www.unixnerd.co.uk/e30problems.html

Those BMWs had a nice classic look to them and were built better back then (and also built in Germany, a plus) but 33 years later I regard them as project vehicles or weekend toys.  If your friend is into cars (esp. classic BMWs) and likes to work on his vehicles and has the budget (because repairs can still be costly) - this could be a fun one for him and worth keeping, but only as a project/weekend car and assuming he can afford to keep multiple cars.  Hopefully, it is not his only vehicle.  It would be a good idea for him to look for another vehicle (and newer too!) to be his daily driver and either keep this BMW vehicle as a project/weekend vehicle (again if he has the budget and can afford to), but if he can only have one car then get rid of the BMW.