WoNdeRIng if iT's worth typing out a reponse

. Thread might very well be closed by the time I post it. Hey, RememBEr whEn wRiting like tHIs was for soMe reasON popular? Glad that's over with... anyhow...
The proper mature way is to not take other people's issues into your own hands and think or believe you can change or save anybody. Triangulating is a word people use for people causing trouble by adding a third person to a situation, when there is no place for a 3rd person. OP has a relationship with the singer who asked him to play. Other bassist is an acquaintance. Acquaintance has a relationship with the singer. 2 separate relationships. It's the singer's responsibility to deal with the other bassist, and the OP. It's the OPs responsibility to deal with the singer and make a decision one way or the other. To try and help other bassist become a better player, to inform him of the singer's intent, to try and manipulate and make things "right" in any way with the 2 others involved is triangulating. What goes on between them is absolutely none of the OPs business. Doing so will not have a beneficial effect. I could spell it all out, but it doesn't take much of an imagination to picture the scenarios should the OP start trying to "fix" the situation.
Bottom line. Singer wants to replace bass player. OP needs to decide whether he wants the gig or not. If he does, IMO and experience, there is nothing immoral or unethical about it whatsoever. It's life, and sometimes people get a little hurt. If they're healthy people they'll learn and grow from it.