All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
Saying that Shaykh Al-Islam Ibn Taymiyyah described the Companion Ibn ‘Umar, may Allah be pleased with him, as an innovator in religion is a heinous lie against the Shaykh . Refuting this lie is easy if you go back to his statements and read them. Perhaps your friends are referring to what he said about intentionally praying in the places where it was narrated that the Prophet prayed during his travel (only because he happened to be there at the time of prayer); and whether or not it is permissible to intentionally set out to those places to pray in them?
Shaykh Ibn Taymiyyah discussed this issue and mentioned that it was not narrated from the Rightly-Guided Caliphs and other Companions that they headed to those places to pray there, and this was not authentically narrated except about Ibn ‘Umar, and that heading to them to pray there became an innovation after the time of the Rightly-Guided Caliphs. So your friends might have understood that Ibn Taymiyyah meant that Ibn ‘Umar was an innovator in the religion, which is not true. There is a difference between ruling that an action is an innovation and ruling that its doer is an innovator.
Shaykh Ibn Taymiyyah said in Iqtidhaa’ As-Siraat Al-Mustaqeem:
“As for intentionally praying in those places where the Prophet prayed because he happened to be there, then this was not narrated by any other Companion than Ibn ‘Umar. Rather, Abu Bakr, ‘Umar, ‘Uthmaan, ‘Ali, and all the other foremost Companions from the Muhaajireen and Ansaar used to go from Madeenah to Makkah for Hajj and ‘Umrah, or just as travelers, but it was not reported that any of them sought to pray at those places where the Prophet had prayed. It is known that if this matter had been desirable to them, then they would have preceded us in doing it, as they were most knowledgeable about his Sunnah and most adherent to it than any other people. The Prophet said, ‘I urge you to adhere to my Sunnah and the Sunnah of the Rightly-Guided Caliphs after me; hold tightly to it. And beware of newly-invented matters, for every newly-invented matter is an innovation, and every innovation is a misguidance.’
Seeking to pray at those places is not the Sunnah of the Rightly-Guided Caliphs; rather, it is an innovation. If the saying of a Companion and his action are contradicted by another Companion, it is not evidence, let alone if it is contradicted by the majority of the Companions.”
Shaykh Ibn Taymiyyah said in another place in the book:
“The second issue is to particularly seek to pray at those places without it being the time for his prayer. Rather, he just wanted to pray and supplicate because of that place. This was not reported from Ibn ‘Umar or anyone else. Although some people claimed that Ibn ‘Umar did that, it was proven that his father ‘Umar forbade that. It was confirmed by Tawaatur (successive mass narration) from the Muhaajireen and the Ansaar that they would not do that. So it is not possible that Ibn ‘Umar had done something that would be evidence against his father and against the Muhajireen and the Ansaar…”
Allah knows best.