CAN I marry a man who had three kids with a woman and never married her?

From a female NIGERIAN

The man I am dating right now was with a woman for 17 years and had three children with her but they never married. He never even proposed to her. We met in April this year shortly after they separated and the reason they separated was because the woman gave him an ultimatum, either he married her or they were over. He chose to leave the relationship but now he wants to marry me. He wants us to have a wedding in December. I was okay with it at first until I received the longest e-mail I've ever read from his children's mother. She explained everything that went down between them and asked me to please reconsider marrying him. She sounded very heartbroken. Should I let him go to return to his family or should I do what he wants and marry him? I'm confused. I love him though but I've been feeling guilty since reading the letter.