How do i make it easier for her when her mom leaves?

Have her mother write her letters and let her write back, let them make phone calls to each other. If you have a webcam and her mother does too then they could see each other that way. If her mom is unwilling to stay in touch though you will have to be very supportive. Tell her that when she's missing her mommy that she can talk to you about it, give her lots of hugs and cuddles. You can't completely make it better but you can be there for her and let her know that you love her and understand that she's unhappy. Let her know that you'll always be there for her.