I woud say there are too many variables to answer that. Marital status, education and so on. If their orders were valid it maybe possible he could be recognized as a deacon and continue on with seminary studies. The best thing to do is to talk to a Vocations director ina dociese and ask questions. They are going to have to know so much more even beyond whether the order may or may not be valid. Someone who is mentally unstable, financially unstable, living an immoral lifestyle, and on and on would likely be screened out no matter what.
Two words…Vocations director. Begin a dialogue and do not expect fast results. Be patient through discernment.
North