Does not informing a person performing ablution over a barrier—while being aware of it—constitute withholding knowledge and abandoning advice, and does it entail a curse?

If a Muslim sees someone who is performing their purification (taharah) or prayer (salat) incorrectly, it is obligatory for him to advise and teach them, because the Prophet, peace and blessings be upon him, said: "Religion is advice." This is part of enjoining good and forbidding evil. So, whoever abandons teaching an ignorant person the obligatory knowledge, despite being able to do so, is sinful and exposed to a warning, and may fall under the saying of Allah the Almighty: "Indeed, those who conceal what We sent down of clear proofs and guidance after We made it clear for the people in the Scripture - those are cursed by Allah and cursed by those who curse." However, repentance nullifies what preceded it, so whoever repents with sincere repentance, Allah accepts his repentance, and he becomes like one who has no sin.