This is one of the most common concerns a parent/carer comes to me with when I get a new student. Here is my response to a parent's recent request for advice and guidance on what implications the differences between the two make.
In short: a real (acoustic) piano is always the ideal! That said, a keyboard is a good alternative when budget/space are limited, and you have the added 'fun' factor of different instrument sounds and drumbeats/demos. And of course the student can plug headphones in so s/he can practise at any time of day/night!
Essentially, however, it is far better to learn piano first, then move to keyboard later if desired. The reason is that when you learn piano and practise on a piano, your technique will be far stronger and more advanced, as the muscles in the hands/fingers will be well-trained to achieve all the necessary agility and dexterity required to play a wider range of styles/techniques. This is to do with how the piano is made - the keys are 'heavier' to press down (properly weighted), and larger, and of course the piano has the foot pedals to enable to pianist to adjust the timbre for expressive purposes. The piano is made from a combination of solid wooden components, metal, felt, and steel. As such, the piano has a completely different feel to a keyboard, which (because it is made purely from plastic and electronics) will tend to have 'spongier' (lighter, less responsive) keys, is usually shorter in length (ie. less/smaller keys), and a far narrower breadth of expression available to the player. It is easier to go from playing the piano to the keyboard, rather than from the keyboard to the piano. It's a bit like if you learn to drive in a manual car, then driving an automatic is fairly straightforward to adjust to, whereas if you learn to drive in an automatic, adjusting to driving a manual car is really hard!
If you have the choice between getting a piano or a keyboard, I would say go for the piano, every time.
The piano I have at home is a beautiful 1980s Silberman upright piano, with a soft but clear tone and perfectly balanced and weighted keys. However, if I won the lottery, I would be going straight to Steinway in London for one of their grands...
I am often asked for keyboard recommendations. Below is a range of keyboards with different functions and price brackets (please note, other brands/retailers are, of course, available):
https://www.dawsons.co.uk/yamaha-psr-e363-portable-keyboard - a basic keyboard, with lots of fun sounds and drumbeats, fun for beginners.
www.gear4music.com/Keyboards-and-Pianos/DP-6-Digital-Piano-by-Gear4music-and-Accessory-Pack/12SU - a great bundle (comes with headphones, an adjustable stool, a sustain pedal etc) and excellent quality weighted keys. Not as many 'fun' features as the basic keyboard above, but this would last the student a good few years past beginner stage and into intermediate.
https://www.dawsons.co.uk/yamaha-ydp-143-digital-piano-rosewood less portable, but an excellent example of an affordable 'proper' digital piano which would take the student well into intermediate level, especially if s/he would like to do grade exams in the future.
Alternatively, if you have the time I would recommend you go into a music shop (e.g. Holmes Music in Faringdon Road, Swindon) to try some out and see what you like.