The first thing is to find out if they are using a wifi router in addition to the one that is part of the Telmex modem.....if not, I would recommend you pick up one (n type) and piggyback it on the modem, and use that for your signal.....that's what the person I'm "sharing" with is doing. They are not that expensive...on the order of $40-50. The built in ones ( part of the modem) usually don't have much range.
Something else to try is moving around in your house and seeing where the best reception is.....and also, if your friends are agreeable, move theirs closer to your house, and up higher if they have a second floor. The high gain directional antenna will work if you have some way of attaching it to your computer (to your wifi card). If you have a built in receiver (like on most laptops), you probably won't be able to.
In terms of range, I'm getting excellent reception from about 150 ft. on the one I'm sharing (separate wifi router).