Some options about how could this go:
> he NTRs any girl in order to keep the friend focused on the demon king hunt
> he NTRs girls that try to use the friend in order to protect him
> he starts to resent the friend and NTR girls just coz
Anyway in the end hes still an asshole, even if he does it to protect the friend he would probably not tell him, the ntr girl would probably go sorry i got to go away or something like that leaving the friend sad
Welp at least for now it looks like hes doing it to "help" him