A couple of things to consider:
Not all chemotherapy causes hair loses.
If chemo does affects hair loss, it may not be the same amount of loss for everyone.
Therefore, I would advise your friend not to purchase a wig until she starts to lose hair.
The cost of a wig may be covered through various insurances.
She may want to investigate this. If she can get coverage, it may only be for a specific amount of money. Therefore, she doesn't want to purchase a $300 wig if only $200 will be covered.
When I worked at a cancer centre, I met many people that wore wigs.
Because they wore their wigs daily for several months, some wigs wore out fairly quickly. Your friend may prefer a wig made from real hair rather than synthetic hair.
You are a good friend - it is great that she has you during this difficult time.