Is it permissible to eat a gift (sweets) from a person who does not pray, knowing that the slaughtered animal of one who abandons prayer is not to be eaten? And does this ruling apply to his wealth and gifts?

There is no harm in accepting a gift from someone who abandons prayer. If it is permissible to accept a gift from a disbeliever, then accepting a gift from someone who abandons prayer is even more appropriate, especially if it leads to a benefit, such as softening his heart and encouraging him to be righteous. The Prophet, peace and blessings be upon him, accepted gifts from kings. Accepting a gift is not like accepting sacrificial meat. The sacrificial meat of someone who abandons prayer should not be eaten if his disbelief is considered a disbelief that expels him from the religion, but the more preponderant view is that his disbelief is a lesser form of disbelief.