there are other similar stories..
and this one reminds me of my character i WoW.. i played as priest, but i had higher dps than anyone in my guild
anyway, in many mangas is basic "heal" something as living magic, so basically hes not even healer, but dps of different job
and that fact he didnt told the previous party... its totally because he didnt trusted them, he even though it wasnt his fault, because they just have fun, instead of fighting seriously.
so no wonder he kept that secret, and to this girl, he has no choice, because he need money to survive, thus have no choice when noone else want him, and maybe he felt he can trust her.