Hello andydassel,
First, glad you are venturing into hydroponics. What is your girlfriend complaining about? It costs to much, it takes too much of your time, where the plants are etc.. What dos she like to eat a lot of? I've found that growing what someone likes a lot, helps generates interest.
Overall the plants seem to look healthy. The purple stems is normal for tomato plants. You'll usually see it on older, thicker sections of the stems. Some curling of the new (and older) leaves is normal too. However if most of the new foliage is growing deformed and/or curling a lot, it could be either from
nutrient strength being to high, or not balanced.
I assume your not in the US (we don't use Euros here). So I don't know if I will be familiar with the type of
nutrients your using. You said it was a 3 part nutrient. How are you mixing them, how much of each part per gallon of water? Are you fallowing manufactures directions? Do you have a link to them? And do they have an online mixing chart?
Browning and dieing of older leaves is normal, however I'm torn because in this case I think it might be an early sign that the roots might be too wet. I say this mainly because the plants aren't that big in the pictures yet, and because your using rockwool as a
growing medium. Also I can't tell if you have good drainage in those containers from the pictures either. They basically look like the rockwol is warped with shrink wrap. While tomatoes are water loving plants and drink a lot of water, the don't like wet feet. So they need good drainage too. If this is the case, the plants look small enough in the pictures that you can replant/pot them
One of the reasons I don't like rockwool that much is it saturates easily, and drains slowly. I don't use it for much other than just small starter cubes. I prefer using coco chips. Coco chips provide really good aeration while still holding moisture, and providing good drainage as long as it's not sitting in water. Coco fiber and coco chips are also cheaper too.
F.Y.I.
If your friend knew how plants actually take up the nutrients they need, he would know how natural hydroponics is. I'm betting that he think it's growing plants in a bunch of chemicals. The fact is plants can't absorb the nutrients they need unless they are actually broken down into a single chemical element (just like the hydroponic nutrients are made of). Even in soil. The only difference is that in soil you need a wide array of microorganisms, bacteria, fungi, micro flora, acids, etc. to break down the organic material into those "same single chemical element" hydroponic nutrients are made of that the plants can finally actually use.
P.S.
As for your contest, I think you have a significant disadvantage. Especially as the plants get bigger, I'm sure only getting light from one will become an issue. Outside you can get both direct and diffused light all day. When only getting light from one side, most of the plant only gets diffused light. Then the more leaves on the plant, the more they will block and shade what little light the plant is already getting. If you could put the plants outside of that window where they can get good light, they'll grow much better.