Is Jane cheating on Stephen?

Define cheating. There was certainly a relationship, but Stephen was aware of it.

Numerous (different) sources describe how Jane and Jonathan had developed romantic feelings for one another but had agreed to not let them destroy the family. This is an extract from the second book linked to above:

...Hawking's personal life had also taken an important turn. Jane confided to him that her relationship with Jonathan had matured from friendship to romance. Nevertheless, she had no wish to disrupt their family. She later described how:

"Stephen said he would not object so long as I continued to love him. I could not fail to love him when he showed such understanding... At the time I felt very guilty but [Jonathan] was a godsend. We were rarely alone together and tried to maintain our code of conduct in front of Stephen and the children, suppressing displays of close affection... Jonathan and I had struggled with our own consciences and had decided that the greater good - the survival of the family unit, Stephen's right to live at home within that family unit and the welfare of the children - outweighed the important of our relationship."

Indeed, all involved were so discreet that no one outside their circle of family and friends knew of this unconventional agreement.

So they allowed some sort of romance to develop between themselves, but they kept it very, very discreet. Then, after the divorce, they became an item and were married.

Whether this can be considered cheating is debatable, since Stephen appeared to be fully aware of it.

The only thing that should be considered is the hurt Stephen would have felt knowing that his wife likes someone else. His own wife made a baby with another guy. If Stephen was not diagnosed with the disease, it is obvious that the relationship between Jonathan and Jane would not have happened.

Just getting involve in a romantic relationship with someone else besides her husband was simple cheating. Eventhough she told him about it and in her own words she is saying he agrees on it, still cheating their own vows of marriage. This is the world we live in today and please, she wrote that in the book as they had to struggle with their feelings because of the sake of the family having they already destroyed the concept itself and cheat their own believe. Nobody is perfect and it is not to me to judge anyone but please, people in modern times should stop finding excuses to justify their actions....At least the movie was a gentleman way to explain the situation without giving you the disgusting details, something she could not do in her own book and in a disrespectful way violate Hawkings private life. Moreover, this so called God believer disgrace God´s name no because of their imperfections but because they try in a way to justify them.