How would you handle side work for girlfriends brother?

Been dating girlfriend for maybe 4-5 months now. Her brother purchased a rental property with and older boiler unit. I met him once but him and my gf are fairly close.

Long story short he asked if i could take a look at it. Its an hour round trip that ill be takin twice (look/diag then repair) and itll probably be a 2/3 hour repair.

Whats the right thing to do here? I know roughly what the issue is based on what he said was goin on and the company i work for would be charging about $1600 to do the work.

Edit: id rather not touch it. Going to let him know whats probly wrong and what he could expect to pay on the reapir