Can a man feel relief after Salatul-Istikhara regarding marriage to a girl, while the girl feels nothing after her prayer?

A vision after Istikharah is not a prerequisite. The prevailing opinion is that a person should proceed with the matter and not abandon it unless Allah diverts him from it. The girl should accept the marriage if she is pleased with his religiosity and character, for Allah will choose what is best for her as long as she was truthful in her Istikharah. If she refuses, there are many others, and perhaps the good lies with someone else.