Restaurant near forest hills for 40 people
There is nothing in the immediate area except for Doyle's which wouldn't meet your "nice food and atmosphere" requirement.
You could take a look at West on Centre in West Roxbury. It has a pleasant function room downstairs. The drawback is that parking is on the street.
Why don't you ask the people in the office at Forest Hills cemetery for suggestions. This must come up all the time. Sorry if I'm assuming incorrectly that the funeral is at the chapel at the cemetery.