It might. I have no idea how they are tested, so I can't say anything about their reliability, etc.
OR you could learn more about herpes and maybe not resort to that. :)
Assuming she has genital herpes type 2 (if she hasn't found out for sure what type she has, she should - she can get a type specific IgG blood test done), then the transmission stats are as follows:
Ghsv2 transmission, female to male, over the course of a year, assuming sex 2-3 times a week:
Only avoiding sex during an outbreak - 4-5%
Adding condoms OR daily suppression (Valtrex or acyclovir) - 2-3%
Adding condoms AND daily suppression (Valtrex or acyclovir) - 1-2%
At best, I'd assume that a sleeve extender would be as effective as a condom, which is just as effective as her taking daily meds to help suppress the virus. If you plan on being monogamous, you can skip the condom (assuming you have both tested negative for other STDs), or just use condoms. I don't have a penis, so I don't know how a sleeve extender feels, but they look like they reduce sensation for the person wearing them more than condoms would.