Hi, I understand your concerns and I think it is wonderful that you are being considerate of your friend's feelings. This shows that you are a true friend. This really can go either way depending on the person and with some people you can lose either way since if you do not invite her she can be offended, but if you do she can be hurt. Therefore I think your best bet is to talk with her in private and be honest about your feelings and the situation. She will need to face mothers day either way and probably is a difficult time for her, so speaking to her about this party will show that you care about her feelings and regardless of what she chooses she will appreciate your thoughtfulness. I would just simply let her know that you are planning a party and would love for her to be there, but are aware of how hurtful it may be, but wanted to give her the option to decide if she wanted to come or not since you did not want her to feel you were excluding her. Of course you can put it in your own words, but that is generally the basis of what I would say it. Just be true and honest then she will feel your sincerity. This way you can not go wrong. I wish you well and please let me know if I can be of further help.
I appreciate your advice. I know there is no easy answer to this one. I appreciate your sensitivity for the situation. Thanks,