It really depends who the third party is. Generally I would look and see which house is symbolized by that type of person (friend, parent, supervisor, &c.). The planet ruling the sign on that house cusp would be the 3rd party's significator. Or sometimes you would use a planet as the ruler: the surgeon is Mars, the professor is Jupiter, the "other woman" is probably Venus. But it gets tricky if you end up with the same significator for two different people.
In your example, merely "talking to" another person doesn't sound significant. So you might use the 5th house (of flirting and casual love affairs) from the 7th house (of your ex,) or radix 11th house. It is ruled by the sun. Mars (him) is past its last aspect to the sun, so I wouldn't think he's chatting her up successfully.