if he is losing pressure only in the kitchen it aint a leak . if it's a leak he will have to reroute all the hot water lines not just to the kitchen
if the pressure relief valve is working a electric water heater will not explode unless it's old a rusted out . then it'll just bust and leak .
imo from what it sounds like the air is coming from the main supply line and some thing is part way stopped up .
How else would he loose pressure if it doesn't have a leak? Its loosing pressure at the sink because its the line feeding the sink... Just like if it were a service line with a significant leak, the whole house would be affected
