I think what you're asking for here is a way to make it obvious to the user that the child record is connected to a deleted parent. Typically what I do is I make a nice clean formula check box field on the parent record with a formula of true called [Donor Exists?]. Then I look that up down to the Child table. If that lookup is false, then there is no parent record.
So one solution is to have a formula Rich Text field on the form telling the user that the donor has been deleted. So the formula for that would key off of [related donor] being greater than zero, but the look up of [Donor Exists?] would be false.
You would have a form rule to only display that warning when that situation existed.
Another choice would be to change the record picker fields or the report being used for the drop-down to have the first field be the related donor. In that case, the red number would show as soon as the record went into edit mode. The problem with that solution is that it's kind of clutter for the 99% of cases where the user is trying to choose a donor, and they are seeing the first column of the record picker fields being a number.
