Is buying a house/condo in Hua Hin a good investment?

I completely agree with your friend and also expect a future price increase, but not so much in HuaHin itself as the supply is still very large there, but in the region south of HuaHin (SamRoiYod).
However, with the opinion “I have to see it all first” you will not be able to benefit from this.

Definitely don't. The houses do not increase in value, as there is so much new construction (competition) (and will come) that an existing house will greatly decrease in value. (From my own experience, the difference between new construction and existing (2/5 years) is no longer so great, which means that new construction takes precedence. It is better to buy land if one of the partners is Thai, because not everyone wants to live in a project.

You only get that increase in value if you do not pay too much for the condo when you buy it.
And that will be difficult.
The owners of the condos have undoubtedly also followed the news, and have probably already increased the sale price of the condos.
You can only benefit from a price increase if you intend to sell that condo at some point.
If you want to live there for the rest of your life, that price increase is of no use to you.

Incidentally, the most important question is how the “Hua Hin later” will be different from the “Hua Hin now”.
Is “Hua Hin soon” a place where you would like to live?

The airport is expanding, the train will take at least another ten years. An express train from Bangkok to Chaing Mai is the first priority. I think Hua Hin will remain interesting. I think it's a nice place and if you're in need of some culture or a city feeling, Bangkok is just a pleasant bus ride away.
Two months ago I bought a villa just outside Hua Hin in one of the many developments. It's a buyer's market so bid low.
My plan is to emigrate in three years and then live in Hua Hin for ten years. Otherwise, you're just throwing ten years' rent away. So if I may lose some money on the sale, so be it.
But my bill was correct now, with the euro exchange rate, villa prices, etc, so I went for it, you can't predict the future.
Good luck!

Best ,

I am a bit of an expert, I bought my first house (new construction) which was well located just outside Hua Hin ten minutes from the city about 13 years ago and sold it about 4 years ago at a loss to build on a large plot land (3.2 Rai) , one house has been completed for almost four years now .
Well I can tell you that buying new construction is not very profitable, there is a lot for sale in Hua Hin, the supply in the second-hand market is so large that it is depressing prices, while new-build prices are rising, but they have a better quality , at least if you don't build too cheaply.
I advise you because of the sharply rising land prices to buy land and build it yourself with local builders, the price you spend will be just as much as buying in a development, with the difference that you have much more land , and much more land also means much more rising value , the built house does not rise , even falls .
You can also buy a house on the second-hand market, you can find real bargains from departing farang, if you buy cheap the price will only increase, just like those people who bought my house at the time see their sales price rise by buying second-hand. you usually buy furniture included, which is an advantage if it suits your taste
I wish you a lot of succes