I try to get clients to book facial waxing on a separate day to a facial if possible. Sometimes the skin is more sensitive after a facial due to exfoliation etc. Client has never had a problem when I suggested this either.
As the others have said I think its probably best to carry out waxing on a different day to the facial treatment as the skin may have an adverse reaction.
If the client really want to have both on the same day you need to let them know of the possible outcome so if they do have a reaction they were aware of the risk. Its sometimes hard keeping our clients happy and balancing that with good practice as they often can make the wrong choices for themselves.
A facial treatment , followed by face waxing and I see you are in Greece so potential sun exposure directly afterwards is not a good combination..