It is true that you could cause some issues running a generator with solar. If you have a large solar output and little load, the inverter wants to push power out onto the grid. If the "grid" is a generator, the inverter will raise the voltage it is supplying to drive to drive it's full power. This increase voltage will cause the voltage regulator (on a better generator design) to back off the output of the generator, since the voltage will be too high. This will all happen within a second or two and the likely outcome is the inverter will shut down due to overvoltage, and restart again. It is unlikely to damage the generator. If there is a reasonable load at the time, they may work well together.
There is a bigger issue. Most grid tie inverters have a very narrow range of frequencies that they will operate with to supply power. On my Fronius 10KW, the range of allowable frequency is 59.3-60.5 Hz Outside that frequency window, it will not sync and connect. Since the generator is quite a bit less accurate in terms of absolute frequency control, it is unlikely to be an issue. I have had several day long runs of outage with my genset running (9.8 KW diesel) and I have left the solar enabled (9.2 KW). First I always have some base load, and I saw that when the solar would connect (when the genset frequency was within the window) it would only last a few up to about 10 minutes, and then go off line again. I saw times when the solar was feeding and a fridge or well would kick on, and the motor starting would cause a momentary dip in frequency and trip the solar off. All in all, on a day where I would normally see 30-40 KWh of solar, I saw maybe 2 KWh of solar supply, supplied a few minutes at a time.
One is certainly safer disconnecting the solar from supplying during genset operation, but with all of the frequency and voltage interlocks on a modern grid tie inverter, it is likely that it may not connect at all, and if it does it will be only for a few minutes when conditions of the gen power and load are just right.
paul