He may wish to consider additional asset protection strategies prior to marriage, such as the use of a trust or entry into a prenuptial agreement.

The issues that come to mind are (a) it won't be apparent from the couple's tax return that you're paying part of the tax bill, (b) taxes on the separate property should not be paid by the marital estate (if they are, at a minimum the wife will get a claim to recover her share of the amount contributed to the tax bill), and (c) active management of the asset during the marriage could potentially cause some portion of the increase in its value to be deemed a marital asset.