بسم الله الرحمن الرحيم
Shias generally hide their true face under the cover of taqayya; therefore it is against precaution for a Sunni girl to marry a Shia boy though he repents from his Shia beliefs and practices. Yes, if he is believed by his friends especially religious and knowledgeable persons to be a true Sunni, then a Sunni girl can marry him. However, one should exercise maximum care in marrying a Shia.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.