Which tube will fit a -(700 x 25c) wheel?
The marking on the tire '700 x 25c' means a 622mm diameter rim, with a nominally 25mm wide tire. ('700c' comes from an older French standard for wheel and tire sizes, it's very commonly used to refer to the standard road bike rims and tire size).

Inner tubes fit a range of tire sizes as they are quite flexible. The two numbers denote the min and max tire sizes. A 700 19-25c tube will fit, as will a 700 25-28c. As long as you tire size fit in the range specified you are good.

Note that tires and tubes may also be marked with 'ISO 622' or 'ETRTO 622' These are both equivalent to 700c.

You also need to get a tube with the correct valve stem length. Tubes come with a range of valve lengths to fit different depth rims. The length is not usually marked on the tube but is easy to measure.

The simplest thing to do of course is take the punctured tube to a bike store and simply ask for new tube that is compatible, for a 25mm tire. Or get your BF to buy his own tubes (or learn how to patch them).