This is a very frequently asked question. Your partner is a first time buyer - that will not change on marriage.
If you buy a house together, then first time buyer relief is not available to either of you.
If s/he buys in her/his sole name, then the relief is available. However, it may be that the borrowings to support the mortgage to buy the house you actually want to buy together will need to have you both involved and again relief is lost. Also, a lot of lenders will insist that a "family home" ( i.e. where a married couple reside) should be in joint names. This is to avoid a situation where , in the event of financial difficulties, both parties are on title and can be joined in proceedings.
Revenue deal with ftb relief on their website.
mf