There are genuine people and business in the online just as there are some frauds and fakes.
So it is our prudence and care and caution that can avoid problems and pitfalls.
In this particular case:
I hope that your friend had not disclosed any confidential matters which can embarrass him if someone else knows it or becomes public. From now he can go with prudence and caution. Let him call the astrologer by telephone(no online call) and get full details of whereabouts. Let the talk be restricted only to assess the consulting place or office and timings and method of payment, booking, direct meeting, timings, etc. If the person answers with clarity and detail and there is nothing to doubt, then you can proceed further. But as an additional caution, your friend can go to the place discreetly and assess himself about the genuineness and reputation of the astrologer. Based on that he can deal or not deal with him.
Now, to tell about the general risks in chatting/talking online; Once we are connected to the internet, then we are pron and vulnerable to any type of risk. There are vulnerabilities for snooping and hacking by anyone including legal agencies. So when talking online be careful to use words which will not bring problems if quoted later, and not at all illegal or prohibited. It is always prudent not to discuss and disclose sensitive and too personal matters. Do not open video record /video call as far as possible to strangers. Available and updated online security software may be installed in the device to the possible extent. These prudent and alert ways generally suffice our normal general care. for online chats and talks.