Sounds like she's looking for something that he's not really ready to provide. You can't really force your dad to open up his feelings. That's something that usually happens when people have to do some immediate introspection due to some grave life circumstances.
If it gets really bad, your mom will start pulling away and that might cause your dad to reflect on himself, but probably not. If your mom is happy enough and finds spiritual/emotional nourishment outside of the marriage (not in a cheating way, but in a healthy way) then no problem. If she doesn't and craves it, maybe it's better that she find a new partner. Either way, I don't think this is something you need to interfere with. It'll work itself out on its own.