I would suggest that if you don't know her very well, you might write her a short letter. Reading it would give her a chance to react in private, rather than with you right there on the phone or the in person. It also would give you a chance to choose your words if she might think you are blaming her son for not telling her.
I didn't need my husband to tell his family, I told them myself.
I would tell her just the same way i told my own mother. i had to beg my fiance to tell his family . i would ask him everyday did u tell ur fam yet