Your friend is not commiting a sin by missing Mass because this priest is an occasion of sin. He obvously has problems and may be abusing other parishoners. For example, he may force other parishoners to go into detail about their sexual fantasies.
I know other posters suggested writing to the pastor, DON’T! This needs to go straight to the diocese. If the pastor is informed, the priest may be spoken to informally and it will give him time to cover his tracks, it will also tell him that your friend reported him. This situation is far, far too serious to be dealt with at parish level.
if your friend will not report him, you must, for her sake. Get this priest out of her life so that she won’t be put off church forever. When writing to the bishop, be sure to mention that your friend will not attend confession at her parish and will not attend Mass without you, as they need to know that this behaviour has resulted in a parishoner being put off attending the Sacraments.
