Advance ticket from Kleine Scheidegg to Jungfraujoch necessary

You can probably get it that morning or the day before and still get a seat. That's why the first train is discounted: a lot of people won't get up so early. Just buy the tickets at the station.

Maybe things have changed in the last year or two but I walked up and bought my ticket just when I was was ready to go up. No trouble.

You do not need to book it in advance - and why would you want to? If the weather is poor there is no point to making the trip up there.

Check the Swiss Panorama web site before you go up to make sure the weather up there is good. It can be nice in the valley and poor above.

Thank you for all your advice.

No, it's not necessary to book the tickets online in advance. If you're staying in Interlaken, you can easily buy tickets at the local station there, even on the day of travel. That particular trip is one of the most expensive in that area, so you might consider buying one of the Swiss Passes, such as the Half Fare Card which will reduce the cost.

Depending on where you'll be travelling in that area, you could also consider something like the Jungfrau Pass.

http://www.jungfrau.ch/en/sommer/tourismus/ausflugsziele/jungfraujoch-top-of-europe/angebote/#jungfrau-travel-pass_1020

Be sure to check the weather on the day you make the trip to the Jungfraujoch, as it will be a waste of time (and money) if the top of the mountain is shrouded in fog and there are gale force winds.

You might pass this information on to your misinformed friend.

I would not consider it necessary to prepurchase the tickets. The station is smack in the middle of town and, especially since you are choosing the early train, you shouldn't have trouble procuring tickets. Great ride and good trip!

Thank you very much for all your advice, I will buy Swiss Travel Pass to save some money.