[3] He is the third and youngest child of Marianne Wetche, a teacher, and Jan Frendrup, a real estate agent and part-time board member of the local sports club Tuse IF.

[9] In the summer of 2017, aged 16, Frendrup was included in the first-team training camp in Austria together with fellow academy players Christian Enemark and Andreas Bruus.

[15] During the 2019–20 season, Frendrup started playing more regularly in the fall due to long-term injuries to starters Lasse Vigen, Simon Tibbling and Josip Radošević.

[22] As the season reached its conclusion, and Brøndby pushed for its first league title in 16 years, Frendrup was sent off in a crucial game against AGF on 20 May 2021.

[26] Frendrup made his European debut on 17 August 2021 in the UEFA Champions League play-off first leg against Red Bull Salzburg, which ended in a 2–1 loss.

[33] He made his debut for Genoa more than a month later, on 13 March, in a 0–0 away draw against Atalanta, during which he went into the starting eleven at right-back, an unusual position for him, which he covered for the injured Silvan Hefti.

[34][35] He made an appearance in the remainder of the games that season, as Genoa suffered relegation, finishing 19th in the league table; three points from safety.

[36][37][38] Frendrup was an undisputed starter for Genoa in their first campaign back in Serie B since the 2006–07 season, covering a variety of positions under head coach Alexander Blessin.

[40] Following Blessin's sacking in December 2022, Frendrup continued to play a key role in midfield under new head coach Alberto Gilardino,[41][42] as Genoa earned promotion back to the Serie A with two games remaining.

He also led the league in balls won (122) and successful tackles (131), receiving only six yellow cards, as Genoa finished 11th in the table, 14 points clear of the relegation zone.

[58] During his time at Genoa, Frendrup solidified his reputation as a versatile and tireless player, leading the league in both balls won (122) and successful tackles (131) while receiving only six yellow cards during the 2023–24 Serie A season.

Frendrup's low-profile, high-impact style of play, characterised by his exceptional timing and work rate, made him a key figure in Genoa's midfield.