Is it allowed to speak to someone who is offering Salah i.e. Telling him for example to come and some you in a certain place after he finishes his prayer, for I think that would distract him from his Salah of which someone told me that it wasn't.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

One should not speak to a praying person without a necessity, and the scholars disliked it, as this distracts him from his prayer. Nonetheless, if speaking to him is for the sake of the prayer itself, then this is permissible and it is not disliked to do so. Ibn Buhaynah said: "Fajr prayer was being called and the Prophet saw a man praying, so he told him: "Are you praying the Fajr prayer four Raka'hs?" [Al-Bukhari and Muslim]

Allaah Knows best.