If a husband has two wives and he allows wife #1 to go see sick mother, can he go and stay with wife #2 on wife #1 night and should he inform wife #1
The Prophet of Allah (Peace be upon Him) said:
"He who has two wives and leans to one as opposed to the other will come on the Day of Resurrection with one of his sides fallen."
[Abu Dawood #2133 & Tirmidhi #1141, & others and verified]
This indicates that the husband must demonstrate justice, fairness and equality amongst all his wives. He is warned of this dire punishment of paralysis and deformity in the hereafter, just as he paralyzed and deformed the rights of one of his wives in this world.
It is unlawful for a man to mistreat his wife in any fashion with abuse, hardships, harassment, undue burdens, insults, beatings, abuse to her wealth and funds, forbidding her from lawful outings, etc in an attempt to force her to pay all that she possesses as ransom to her husband so that he may release her through divorce.
Islamic laws do permit the husband to impose certain restrictions upon the wife that displays some immoral and shameful conduct, dishonorable to him and his family, and harmful to the entire society and social order. The purpose of these restrictions is to seek her to return to proper behavior. Those who continue to act indiscreetly, leading to suspicion of actual infidelity may be offered divorce, just as she may seek "Khul" wherein she asks for dissolving the marriage contract due to his misbehavior.
Of course he can and no he does not need to inform the first wife.
But remember he must be fair and equitable between the two of them.