I second (third? fourth?) the advice about neutrally telling them you've received a complaint, cops may get involved etc. And then watch how they behave--let the tenant who complained know that you're looking out for him and you want to know if the behavior improves (and that you totally condone his calling the police next time). A huge red flag, obviously, would be if they complain to your other tenant or make nasty remarks about him having complained.
And then see if they behave themselves. Noisy fights would not be the only problem to be on the lookout for. Give it until it's time for lease renewal. You've got a "great tenant" vs. some new folks who get in screaming matches, and a golden opportunity to avoid the hassle and expense of eviction by simply not renewing the lease. Pay close attention over the next month and see if these are really people you want to keep renting to, vs. if they are making things so annoying for your "great tenant" that you risk losing him. A great tenant in a multi-unit can become almost like a free on-site property manager, letting you know about problems ASAP so you can fix them.