None.

In fact, your picture looks like a leggy tomato that maybe isn't getting enough sunlight. You need all the leaves you can get.

Agree. Ignore the friend's advice. In your photo, if they were right, the one you would have to cut is the tall center one of the 3 in your photo. And that would be the end of your plant's growth and production.

In the jpeg you posted the tiny red one can be cut off under certain circumstances - it is a choice, not a requirement - but you get more fruit if you do not cut it.

Dave

You cut "suckers" - in an operation called pruning - when you have a "jungle."

It will help air circulation, and avoid diseases.

See:
- Show off your tomato "jungle" ! [2014-2015] and

- Show off your tomato "jungle" ! (2016) [Part 1]

Pruning "suckers" (which is a dated and inaccurate term because they don't "suck" anything from the plant) is highly controversial.. Some say YAY others say NAY - I'm in the nay crowd lol.. They're actually called lateral shoots and they grow at auxiliary sites; auxiliary sites can be found at every intersection between the main stem and a compound leaf stem. Some pruning does need to be done (as Daniel mentioned) to keep the interior of the plants from getting too crowded which will promote certain diseases, but over-pruning the laterals will result in decreased yield.. If you have 20-30 plants then you wouldn't notice as much, but if you only have a half a dozen or so plants, pruning laterals will very noticeably decrease your yield. Do a little more research on your own to decide which way you'd like to go. .And remember, you could prune ALL laterals or you could prune NONE - your style of gardening is your choice, and often an eclectic school of thought is advantageous.