Is it permissible for a traveler to pray Maghrib alone and then pray 'Isha in congregation if the time for Maghrib has passed?

If you two are travelers, it is permissible for you to delay the Maghrib prayer and combine it with the 'Isha' prayer by way of jam' ta'khir (delayed combination). It is considered performance [of the prayer] as long as you intended to combine them. However, if the time for Maghrib passes without the intention to combine, then it is considered a qada' (missed, to be made up) prayer.

As for what your friend mentioned, that it is not permissible to make up the Maghrib prayer in congregation, that is incorrect. The Sunnah (definition) has indicated the permissibility of making up prayers in congregation, as the Prophet (peace and blessings be upon him) and his Companions did when they missed the Fajr prayer during one of their journeys and made it up in congregation. Scholars have agreed on the permissibility of congregational prayer for qada', and there is no difference in this regard between making up one prayer or more.