Wow, this is a tricky one. I have to agree with dstinationdrmr, don't wait cause if they both buy tickets to go then its going to become a strenuous situation if they are now out of money because the issue was not addressed. If you can, simply don't invite him, but if you must invite him explain that the invitation is for one, you simply can't afford it, or you can just be upfront and explain you simply don't want her there. Good luck!