First check the guardianship order to see what rights it gives the guardian. You should consult with a lawyer to see what your state's laws say about marriage by a person under guardianship. Ultimately, if you can't find a way to get married, the person under guardianship may have to petition the court to modify the order or appoint a new guardian. The person could specifically ask the court for permission to marry, pointing out the factors you have set out in your question. Good luck.
I hope this helps. If you think this post was helpful, please check the thumbs up (helpful) tab below. Thank you!
My comments are not intended to establish an attorney-client relationship, are not confidential, and are not intended to constitute legal advice. Proper legal advice can only be given by an attorney who agrees to represent you, who reviews the facts of your specific case, who does not have a conflict of interest preventing the representation, and who is licensed as an attorney in the state where the law applies.