Per the wording of the spell -
Choose up to five creatures within range that can hear you. For the duration, each affected creature gains 5 temporary hit points and has advantage on Wisdom saving throws. If an affected creature is hit by an attack, it has advantage on the next attack roll it makes. Once an affected creature loses the temporary hit points granted by this spell, the spell ends for that creature.
Since temp HP doesn't stack, my friend argues that any new source of temp HP would end this spell. I argue that the spell, which is rather weak for third level, is meant to work together with other buffs, and one could apply a better pool of temp HP from a different spell or ability to improve its duration. Otherwise, it amounts to (in my eyes) a less useful Beacon of Hope, and functionally non-castable if a player has any other source of temp HP already applied. Are there any explicit rules for this?