Can I use the sim-free in the Philippines?

If you buy an unlocked phone directly from an Apple store or Apple.com it will work in the Philippines. If you buy it from any other Apple seller it will be locked to a carrier. So your buyer should buy only from Apple, no matter what lies other salespeople tell them.

Your problem, however, is that if it ever needs to be fixed it will have to be returned to the US, as neither the warranty nor out of warranty service can be performed outside of the country where the phone was sold.

That would be the best thing to do. It will cost more, but there will be much less hassle. And the cost difference may not be as much as you think, when you add in the shipping costs, and the fact that the listed price in the US does not include sales tax, which can add between 7% and 15% depending on where the phone is purchased.

If it is an unlocked phone you may be able to use it. It would depend on what bands are used in the Philippines. Check with the carriers in the Philippines and see what model of iPhone they support. Also, understand that the warranty for a US phone is only valid in the US. The only place to purchase an unlocked phone in the US is at an Apple Store as well.

Does Philippines have a value added tax? That’s one common reason for a large price difference between the US and other countries. There may also be import duties that are imposed on the manufacturer.