a muslim accepted that your mother would never convert, and they have been happily married for a long time, his parents should have seen how happy your Mother makes your Father and give them their support and blessing.
It's difficult not to judge but they are set in their ways, old habits die hard.
Your Father could try talking to them when he goes in July and as Gypsy says, if this doesn't help then maybe give yourself a little time and go there yourself, however, it may not be any different, just be strong and be prepared for any outcome.