The first thing I would do is ask about their income. If one of them has a lot more income than the other, it could benefit the two of them to get married.
If you have a tax program, I would collect as much information as I could and do a scenario with both of them as Single and another with their being married.
I would do some scenerio's because sometimes Married Filing Jointly income tax brackets and standard deduction are not always equal to twice the single income tax bracket and standard deduction. Under current law, the marriage penalty is partly helped because the lower income tax bracket (10% and 15%) and the standard deduction for MFJ are exactly double that of single individuals.
MFJ - Married Filing Jointly