I don't think there's anyway to really know just how likely it is the partner contracted HSV. There are likely only a few days a year when the virus is active without symptoms (called asymptomatic shedding or rectivation) but there's no way to predict just when these days occur.
Taking Valtrex daily has shown to reduce transmission among discordant couples (one partner has genital HSV, the other does not) by about 50%, but it still doesn't address your situation and the "how likely?" question.
Discussing herpes with a partner can be difficult, for sure, but it's a good idea and we have a section of our Web site designed to help with this. For more go to http://www.ashastd.org/herpes/herpes_em ... onship.cfm