Możesz to zrobić poprzez odwołanie się do index-u elementu <option> listy <select>
Przykład
<select class="custom-select col-6" name="typ_instal" id="typ_instal" size="4">
<option value="0" selected>Lorem A</option>
<option value="1">Lorem B</option>
<option value="2">Lorem C</option>
<option value="3">Lorem D</option>
</select>
<button value="0">A</button>
<button value="1">B</button>
<button value="2">C</button>
<button value="3">D</button>
select#typ_instal {
display: block;
width: 100px;
}
select#typ_instal option:disabled {
color: rgba(0,0,0,0.2);
}
button {
display: inline-block;
margin-top: 0.5em;
}
const select_ = document.querySelector('#typ_instal');
[...document.querySelectorAll('button')].forEach(button => {
button.addEventListener('click', (e) => {
const toggle_boolean = (select_.options[e.target.value].disabled) ? false : true;
select_.options[e.target.value].disabled = toggle_boolean;
})
});