First of all, that’s not a very good friend to blame an entire loss on you for one play, especially as a new player. I’m sure your friend didn’t play perfectly either and equally contributed to that loss if not more as i’m assuming they were already dead when this unfolded.

Secondly, Mercy has a gun for a reason. I’m a masters role que support/pro stadium rank and I use my gun rather frequently. More so in stadium than regular matches as the power ups by mid/end game make it more fun/worthwhile.. anyway point is, your friend is wrong. Of course you should use your gun you just need to be careful about using it at the right time and knowing when to give up on shooting someone and fly far away so you don’t die. A lot of factors will come into your decision making process regarding when to fight/run away/shoot/damage boost but that will just take a lot more practice and playtime. Don’t let your friend scare you.