My friend watches his 2 sons while living at his mother's house. The boys are there week days and go with their mother on the weekends. The grandmother is wanting to claim the boys on her taxes since they are there most of the time. What does she have to do legally to claim them? If both mother and grandmother claim the boys will grandmother have to prove that she pays for the majority of their living costs? What would she have to do to prove it or what are some ways she could?

Generally, it is the custodial parent who is legally able to claim his or her child as a dependent.

About.com: http://www.irs.gov/pub/irs-pdf/f8332.pdf (http://singleparents.about.com/gi/o.htm?zi=1/XJ&zTi=1&sdn=singleparents&cdn=parenting&tm=188&f=00&tt=12&bt=1&bts=0&zu=http%3A//www.irs.gov/pub/irs-pdf/f8332.pdf)

The custodial parent is generally the parent with whom the child lived for the greater number of nights during the year. The
Noncustodial parent is the other parent. If the child was with each parent for an equal number of nights, the custodial parent is the
Parent with the higher adjusted gross income.

What does she have to do legally to claim them?
In my point of view - nothing.

She has the right to claim them, but if the mother claims them too, the IRS will ask for proof after the fact from both of them. She and the mother need to sort this out first. Proof would usually be the custody papers showing that her son has been given custody of the boys during the week and the support arrangement. If there is no court order, he should get one.

She has the right to claim them..
Would you like to give us the source?