It's a generalisation - if you look at seats which span all stages that have been tested by ADAC they tend not to do very well. Actually anything that combines a booster with a harnessed mode tends not to do that well. It's just a contradiction in terms because you need a harnessed seat to be sturdy and strong which generally involves weight and stability whereas a booster seat should be lightweight and flexible.
I wouldn't have safety concerns about the seat just used up to 18kg.
However, there are a number of issues which I'd point out before you buy it.
Firstly it's rear facing only to 13kg. About 75% of children will be at least 2 years old by the time they get to 13kg, but you can rear face longer with a seat which goes up to 18kg or 105cm (which is about 4 years).
It must be fitted with top tether and isofix - your friend would need to make sure she has top tether, it's less common than isofix. In addition, the tether has to be fitted every time if you want to use the rotation feature, which could be an issue, and it would be quite tempting to just go oh let's not bother with the tether - which would be really dangerous as that's what prevents the seat from moving forwards in a crash.
The fitting instructions are quite complicated and not very clear. It looks like it uses isofix + top tether OR seatbelt only in rear facing mode, and isofix + top tether + seatbelt in forward facing mode. Because of the complexity and the instructions being confusing I'd say there's a chance of misuse.
You can only use the rear facing position in the most reclined seating position. That's quite frustrating for a lot of toddlers who would prefer to sit up so they can see out of the window. However, it's also common among Group 0+/1 seats in the lower price bracket so this may be difficult to avoid. Plus as this seat has a lower rear facing limit, it might be less of an issue, but I know my DS wanted to be more sat up by 10 months, and a year certainly.
It looks like there is only one position for forward facing too, which might be too upright for a child wanting to take a nap.
I'd suggest you look at/compare the following in the same price bracket:
Joie Stages - belt fit, RF to 18kg
Britax First Class Plus - belt fit, RF to 13kg
BabyAuto Dupla - isofix + seatbelt, top tether, RF to 18kg / FF harness to 25kg
Or even a combination of two e.g. Joie Juva + base (£85) and then Britax Eclipse (£60)
None of those are rotating seats. Joie 360 Spin is a cheapish rotating seat which would tick all your boxes, if you can find it on offer it commonly goes down to about £160 in sales which might be doable if you combine with voucher codes, especially if you have until December to buy it.
Osann One I am not keen on because there was a safety question mark over it as a model. Apparently only from one supplier, but still.
There is a Bastian model (I forget the full name) which is a cheaper spin one which also cannot be spun with the top tether installed.