Is there anything in this particular plan document that says a loan becomes payable in full immediately upon the employee becoming a union member (or more generally, transferring to an excluded class of employees)? Usually I would only see that kind of provision apply upon termination of employment, but I suppose it could happen.
Absent that, I don't think so. The employee continues to repay it through payroll deduction (assuming that's what the loan policy says). Transferring to an excluded class means you are not entitled to future contributions. Loan repayments are not contributions.