treat someone to something—supply food, entertainment, etc. at one’s own expense to a person:
- He told me that I was the only one to ever treat him to dinner, aside from his father. Isn’t that sad?
treat someone with something—
1. act or behave towards a person in a manner specified:
- They treated me with a respect that I am still unsure I deserved.
2. = treat someone to something:
- They treated me with a big steak dinner with all the fixings. It was great!
Note: Neither expression correlates in meaning with the phrase treat someone for something—give medical care to a person who has an illness or injury:
- Women who had been treated for breast cancer faced the risk of a local recurrence.