Most Helpful Girl
It isn't wrong for her to "force that decision on him" since he has the choice to leave the relationship. And honestly he probably will end up leaving her.
It is definitely cheating if he sleeps with another girl. That's being intimate with someone besides your partner. If he's even having thoughts like that tell him to dump the girl because he won't make it the 4 years or so it'll take for marriage1THIS IS NOT RELEVANT ANYMORE