Everything Ziel said is very good, but I’d add a few things.

Some multi-filaments are much more durable than others in my experience. I’ve found Wilson Sensation to be very prone to snapping, while NRG-2 by Technifibre lasts a long time and just shreds like natural gut.

Thin gauge is very important. I’ve never used a 16 gauge string that I was happy with, even natural gut. While it’s true that 17 gauge natural gut is the best string I’ve used, I’d rather use a thinner NRG-2 synthetic than use a thick gut. I’ve tried 15 gauge gut (Babolat Tonic with Longevity) and it was rather stiff.

String tension is very important. The lower the tension, the less shock is transmitted.

A light stiff racquet like the Pure Drive can cause elbow paint more readily than a heavy racquet that is flexible because there is less mass to absorb ball shock and a stiffer racquet dissipates less energy, transmitting more into the ball but also into the body.

Your friend can add lead to the handle of the racquet to increase its mass. Physicist Wilmot McCutchen at has said that putting mass in the handle to make a racquet head-light makes things easier on the elbow. The lower the center of percussion is, the less stress is put on the body. McCutchen said that by tail-weighting (with the weight beyond the butt cap) a stiff light racquet he was able to make it safer for the elbow than any racquet on the market at the time.

A powerful racquet like the Pure Drive, if strong at a low tension with a powerful string like thin natural gut can be hard to control for some people, depending upon how they produce their shots. Some players like a tight string tension. Your friend may have to switch to a less powerful racquet if that’s the case. However, some soft strings are less powerful than gut, like polyolefin. Polyolefin is the softest synthetic material available. It’s not known for being very durable, though. There are also less stiff racquets available, and Pro Kennex makes a Kinetic series of racquets with lead ball bearings in the frame to absorb ball shock.

This site lists the stiffness of different strings:

Natural gut. Nothing beats a full set of natural gut. Until you look at the cost.
Aside from that, look for soft synthetics (mostly multifilaments). Here’s the listings for soft, arm-friendly strings from tenniswarehouse:

The softer and thinner the string, the more comfortable it’s going to be. But it’ll also break pretty quickly, too. That’s why these softer strings are often used in hybrid sets, with a more durable string used for the mains.

Please don’t use polyester, it will absolutely have a field day on your elbow. Try something nice and soft.
1.Wilson NXT
2.Babolat Xcel
3.Prince Synthetic Gut
4.Wilson Sensation
5.Babolat Addiction

Babolat Pro Hurricane…excellent at 57