Going to miss his Mom, Do I have to give her back?

For the last two years, every birthday his family or mine we all got together for a dinner at the b-day's favorite spot.  Friday is his Mom's birthday!  Over the course of our relationship she and I have become soooo close!  I lost my grandma (that raised me) in 2004.  This lady so filled that void in my life and is a "true friend" but she is HIS mother!  I am taking her out to dinner tonight, just she and I.  She know everything about what has happened and even about his drugs.  He knows she knows about the cheating (of course they are "just friends" according to him and he thinks she believes it) NOT! She knows it all and encourages me to live my life (but I know she still hopes we will work it out).  She tells me to date and is happy for me when I meet new friends but... she still hopes I will be there for him in the long run.  I told her I can not promise that.  She understands.  I know it is not good for me to continue to see her, digs up memories but I love her for her, nothing to do with him.  I never want her to think I was a pretend friend!  She is turning 73 and is a widow!  Can I keep her? Please!! I need her!!