He wasn't "invited" to a prostitute exactly, and the guy isn't his friend or really a prostitute himself (revealed at the end). At the end of part 1 the son sees his mom with the guy and gets suspicious since she hasn't visited him at work in over a month (Arubain says in the afterword in part 1 https://e-hentai.org/g/2559647/30a1343736/ that the entire setting was inspired by eating at a restaurant and hearing one of the servers complain about how his mother reserved a table at their restaurant again). At the start of this the guy says he noticed someone was stalking them, initially thinking it was some private detective investigating her cheating, and brings him up thinking it's someone who knows her. After he finds out it's her son he has him stay around up to see what his mother is up to, and forces the son to join in on the sex through blackmail. They continue their 3P adventures as the mom has been turned (willingly) into an onahole by now, but eventually she finds out she's pregnant and tells the guy, who then blocks her number and ghosts her. Turns out his hobby was turning women into "onaholes" (in part 1 he says one of the requirements of being an onahole is raw sex/creampies) to impregnate them, and then as soon as they're pregnant he blocks their numbers and ghosts them entirely. In the end the son tells his mom he'll stay by her, and the mom divorces the father (who essentially abandoned her and left her sexless causing her to look for dick elsewhere) and aborts the guy's baby (it could also be the son's since he also creampied her, but it doesn't say anything about whose it is). So after being abandoned by both her husband and fuckbuddy, she now lives with her son who takes up the role of consoling her heart and body (through sex).
I was expecting a son to take over as a sexual partner but the son is still in college while the mother is a sex addict. If left alone his mother will find another man