There are pre-amps that support balanced (XLR) outputs, which may be single-ended, circuitry-wise, internally, and then there are end-to-end, fully-balanced pre-amps. If you want a fully-balanced pre-amp it's going to be expensive, even used, since Emotiva discontinued the XSP-1. If you develop a hotlist of models you're looking for and watch Audiogon carefully you can sometimes find some examples under $1500, but they're likely to be rather old products, which could develop problems in the near future, and be expensive to repair.
Frankly, I think a fully-balanced design is overkill in this price range, and I'd recommend something new and reasonably priced, like the Parasound Halo P5. The Benchmark DAC1-HDR, which I use in a very high-end system, is only $1600 brand new. The Benchmark is not a fully-balanced design, but it measures better than many fully-balanced preamps and has balanced outputs.
Unless your friend's amps are a significant distance from the preamp balanced cables will likely have inaudible benefits too. For runs of less than ten feet any single-ended RCA cables will work just fine. If he can go without balanced cables he might find the Outlaw 975 pre-pro used as a stereo pre-amp might be all he needs at $550. It apparently measures incredibly well. I use a 975 for a small AV system, and while it is a bit ergonomically challenged in the remote control, the price is right and the build quality is pretty good.
None of these suggestions are likely to compromise the sound of any amp in any audible way.
I'm sure someone will pop in here and recommend your friend buy a cheap receiver with pre-outs and use that. Not me, but I'm sure someone will.