I've been using a Sprint MiFi 2200 with my MBP since Sep '09 and I've never had a problem with it. There's certainly no compatibility problem that I've encountered.
What sort of problem is your friend having? She can't connect to the wlan or she can't make the device connect to the WWAN? Is it possible to be more specific regarding the symptoms?
I've even set up my iPhone as a client so that I can download app installs that'll only install via WLAN. No problems with that, either.
Unlike the Verizon MiFi implementation, the Sprint version never needs to connect to a client machine via USB, so there really shouldn't be any opportunity for incompatibility since 802.11 is 802.11.
John