Is it considered adultery if a man consummates his marriage with his wife, whose marriage contract was solemnized during the Friday sermon, knowing that the contract is void?

What you mentioned about the invalidation of the marriage contract after the call to Friday prayer and during the sermon is incorrect. The four Imams have agreed on the validity of the marriage contract after the call and during the sermon, differing only on whether it is permissible, forbidden, or disliked. And whoever consummates his marriage after the call is sinful for being preoccupied from answering the caller to Friday prayer, but this is certainly not considered fornication. Perhaps the questioner confused the ruling on selling with the ruling on marriage. It is selling that is prohibited from the beginning of the second call, and it is not valid if it occurs, according to the preponderant opinion of the people of knowledge. The evidence for this is the Almighty's saying: (O you who have believed, when [the adhan (definition) is called] for the prayer on the day of Jumuʿah, then proceed to the remembrance of Allah and leave trade) [Al-Jumu'ah:9]. This is a prohibition of selling, and a prohibition necessitates invalidation.