My knowledge of machines is a few years old now.
There are some very good suggestions already here about brands. The biggest thing is for her to know WHAT she is planning on using the machine for, and chose a machine that will do the job. Basic clothing construction, decorative stitches, heavy fabric, light weight, etc. Then pick a model of machine that will do that. A machine that sews light weight fabric might not handle denim or canvas for example.
The decorative stitches a machine can do can vary also. Most of the stitches built into a machine are variations of a zig zag. Back and forth in different patterns. If a true stretch stitch is needed, she needs to know if the machine can do that. There are a few other stitches that require forward and backward stitches as well as the back and forth to create. The blanket stitch is one. If she wants to use those for applique, for example, then she needs to make sure her choice can do that.
Stitch width in the decorative stitches can also vary by manufacture.
The store she buys it from is also important because they are the people she will be dealing with for instructions and repair if there is a problem. Repair costs can be huge, she needs to be able to use the warranty that comes with the machine. If she buys from a store a long way away, the local store may not honor the warranty.
The new machines are computers also. Good and bad.

The decorative stitches a machine can do can vary also. Most of the stitches built into a machine are variations of a zig zag. Back and forth in different patterns. If a true stretch stitch is needed, she needs to know if the machine can do that. There are a few other stitches that require forward and backward stitches as well as the back and forth to create. The blanket stitch is one. If she wants to use those for applique, for example, then she needs to make sure her choice can do that.
Stitch width in the decorative stitches can also vary by manufacture.
The store she buys it from is also important because they are the people she will be dealing with for instructions and repair if there is a problem. Repair costs can be huge, she needs to be able to use the warranty that comes with the machine. If she buys from a store a long way away, the local store may not honor the warranty.
The new machines are computers also. Good and bad.