What can my father in law do about his plantar faciitis pain?

Stretch : Plantar fasciitis usually goes away 90% by consistent stretching. Stretch or find a physical therapist that can help you with a stretching routine. The most common stretch is called the "runner's" stretch or "wall stretch". If pain is chronic, you may have other pathology rather than just tight/short ligaments and tendons. Hope this helps.

NEED FOR SUPPORT!: Most shoes aren't always what is bad, usually it's the crummy insoles they come with! there are many possibe reasons (plantar fascitis leaps to mind) that you may have pain in the heels, but try arch supports like spenco polysorb (http://www.Spenco.Com/products/footcare/poly-sorb) and otc anti-inflammatories (like aleve). If these don't help, see a podiatrist.

See below. : Depends upon severity. If mild, and tolerable consider inserts in shoes/sneakers. Antiinflamatories may also benefit at this time ( ie NSAIDS), but consult your pcp for dosage and how long to continue their use as gastric ulcers/bleeding may occur. The pain could take 4-6mos to improve. If worse, consult your pcp for surgical/podiatric evaluation as more aggressive steps may need to be taken.