That isn't the efficiency, which is a number somewhere under 23%, dividing the number of joules of light into the panel by the number of joules of electrical power out of it. Maybe you mean that at peak he gets 91% of the advertised 6kW, i.e., 5500W?

Anyway, you said they paid €7000 and get back €500 per year, which is enough to calculate the ROI at 7.1% per year.

Powerwalls are indeed extremely overpriced. But battery prices are down to below US$70/kWh (US$19/MJ, €17/MJ); if we assume the maximum production in a day is about 25%, that's 36 kWh (130MJ), so it would be about US$2500 for that amount of battery. But probably even €500 of battery (7.2kWh) would make a big dent. That would be 1800 watts for four hours in the evening.

Depending on the type of evening consumption, it might be possible to ameliorate it dramatically with various kinds of thermal storage, or even plugging the washing machine into a timer. Those are much cheaper than batteries.