I've found that it's generally considered "cool" to dog on Trek. In researching actual part vs. part, warranty vs. warranty the Trek line stands up to anything out there at comparable prices. Many people like to say that Trek shoulda stuck with road bikes, that's fine too, to each their own.

The most common complaint I've heard is that people don't seem too fond of the shorter top tube's typically found on Trek's MTBs. Now that can be a ligitimate complaint depending on the rider. If it fits your girlfriend and she's comfortable then that certainly doesn't apply.

I have a Trek and considered all the others first. I was really torn between my Trek and a Specialized but in the end went with Trek because it fit me well and I liked the LBS where I was going to be purchasing it.

Anytime someone states that an entire line of anything is crap you should generally take it with a grain of salt. Trek makes cheaper China bikes (like mine) and they make very nice bikes (like the 8000 series) all the way up to the elite that are stellar with a pricetag to match. (Speaking exclusively about hardtails here) I've never heard a single safety concern with Trek when you're using their bikes as they were designed. Your girlfriend should ride and enjoy her new bike with no safety concerns that are legitimate. People are snobs about different things, to each their own.